Aircraft mounted gas turbine engines are usually capable of operating satisfactorily in rain, hail or snow. Water ingested by the engines under these conditions is usually vaporized during its passage through the engine's compressors. Consequently air which eventually enters the engine's combustion system is generally free of significant quantities of liquid water.
There are, however, circumstances in which ingested water can prejudice the effective operation of the engines. The most common of these circumstances is when the engines are passing through very heavy rain, hail or snow at minimum idle, that is at their lowest flight power setting. This typically occurs during the descent of the aircraft powered by the engines prior to its landing. Under these circumstances, it is well known that the ingested water can have up to three adverse effects upon engine operation:
(a) The engine can progressively run down until it eventually stops. This is a characteristic of older design engines having hydromechanical fuel flow control mechanisms although not of more modern engines having electronic control mechanisms. PA1 (b) The engine can suffer from a flameout. Thus the presence of liquid water within the engine combustor together with the low combustion pressures and temperatures associated with minimum idle conditions result in combustion being extinguished. PA1 (c) The high pressure compressor of the engine can surge and thereby cease operating. This arises as a result of the working line of the compressor being raised by water evaporation and the extra work done by the compressor on the water.